Superabrasive cutting elements in the form of Polycrystalline Diamond Compact (PDC) structures have been commercially available for almost four decades, and PDC cutting elements having a polycrystalline diamond table formed on the end of a supporting substrate for a period in excess of twenty years. The latter type of PDC cutting elements commonly comprises a thin, substantially circular disc (although other configurations are available), commonly termed a “table,” including a layer of superabrasive material formed of diamond crystals mutually bonded under ultrahigh temperatures and pressures and defining a substantially planar front cutting face, a rear face and a peripheral or circumferential edge, at least a portion of which is employed as a cutting edge to cut the subterranean formation being drilled by a drill bit on which the PDC cutting element is mounted.
PDC cutting elements are generally bonded over their rear face during formation of the superabrasive table to a backing layer or substrate formed of cemented tungsten carbide, although self-supporting PDC cutting elements are also known, particularly those stable at higher temperatures, which are known as Thermally Stable Polycrystalline Diamond, or “TSPs.” Such cutting elements are widely used on rotary fixed cutter, or “drag,” bits, as well as on other bits and tools used to drill and ream subterranean formations, such other bits and tools including without limitation core bits, bi-center bits, eccentric bits, hybrid (e.g., rolling components in combination with fixed cutting elements), roller cone bits, reamer wings, expandable reamers, and casing milling tools. As used herein, the term “drill bit” encompasses all of the foregoing, and equivalent structures.
In the formation of either type of cutting element, a catalyst is usually employed to stimulate diamond-to-diamond bonding of the diamond crystals. Unfortunately, the presence of a catalyst in the diamond table may lead to thermal degradation commencing at about 400° C. due to differences in the coefficients of thermal expansion (CTEs) of the diamond and the catalyst, and commencing around 700-750° C. due to stimulation of back-graphitization of the diamond to carbon by the catalyst. Such temperatures may be reached by the cutting edge of a PDC cutting element during drilling of a formation, despite the use of drilling fluid as a cooling agent and despite relatively rapid heat transfer into the diamond table, the substrate and the body of the drill bit on which the cutting element is mounted.
It has been recognized in the art that removal of the catalyst from the cutting surface of the diamond table, particularly at the cutting edge thereof and along the side of the diamond table proximate the cutting edge and extending toward the substrate, reduces the tendency of those portions of the diamond table to degrade due to thermal effects. Consequently, provided the depth of removal of the catalyst is sufficient, the life of the diamond table is extended. The recognition of the aforementioned thermal degradation effects and how and from what portion of the diamond table the catalyst may be beneficially removed is disclosed in, among many other documents, Japanese Patent No. JP59-219500, as well as in U.S. Pat. Nos. 4,224,380, 5,127,923, 6,544,308 and 6,601,662, U.S. Patent Publications Nos. 2006/0060390, 2006/0060391, 2006/0060392, 2006/0086540, and PCT International Publication Nos. WO 2004/106003, WO 2004/106004 and WO 2005/110648. The disclosure of each of the foregoing documents is hereby incorporated herein in its entirety by this reference.